An electric generator produces electricity according to the principles of generator action of a dynamoelectric machine, in response to a turning torque provided by a combustion or steam-driven turbine. The generator is a mechanically massive structure and electrically complex, with typical output power ratings up to 1,500 MVA at voltages up to 26 kilovolts (kV).
Conventionally, the electric generator comprises a rotor carrying axial field windings (also referred to as rotor windings) for producing a magnetic flux field in response to an input current, which is typically direct current supplied from a separate exciter. One end of the rotor shaft is drivingly coupled to a steam or gas-driven turbine for providing rotational energy to turn the rotor. Rotation of the rotor within stationary stator windings (also referred to as armature windings) causes the rotor magnetic field to induce an output current in the stator windings.
As shown in FIG. 1, conventionally an electric generator 10 comprises a rotor 12 carrying axial field or rotor windings 13 for producing a magnetic flux field that rotates within a stationary stator 14. One end 15 of the rotor 12 is drivingly coupled to a steam or gas driven turbine (not shown in FIG. 1) for providing rotational torque to turn the rotor 12. An opposing end 16 is coupled to a separate exciter (not shown) for providing direct current supplied to the rotor windings 13.
The stator 14 comprises a core 17 including a plurality of thin, high-permeability circumferential slotted laminations placed in a side-by-side orientation and insulated from each other to reduce eddy current losses. Stator coils 18 are disposed within inwardly directed slots of the stator core 17, and interconnected to form one or more closed-circuit stator windings. Rotation of the axial field windings causes the magnetic field produced thereby to induce alternating current in the stator coils 18. The generated current is carried to the main leads 19 for connection to an external electrical load. Three-phase alternating current is supplied from a generator having three independent stator phase windings, formed by appropriate interconnection of a plurality of stator coils 18, and spaced at 120° around the stator core. Single-phase alternating current is supplied from a single stator coil extending 360° around the stator core.
The rotor 12 and the stator 14 are enclosed within a frame 20. Each rotor end comprises a bearing journal (not shown) for mating with bearings 30 attached to the frame 20. The rotor 12 further carries a blower 32 for forcing cooling fluid through the generator elements. The cooling fluid is retained within the generator 10 by seals 34 located where the rotor ends penetrate the frame 20. The cooling fluid is supplied to coolers 36 for releasing the heat absorbed from the generator components, after which the coolant is recirculated back through the generator elements.
FIG. 2 is a cross-sectional view of the stator 14, illustrating a face 60 of one stator core lamination and inwardly directed slots 62 carrying a top coil 18A and a bottom coil 18B. The individual core laminations are coupled by clamp structures 64 to form the stator core 17.
FIGS. 3A and 3B illustrate one end of the top coil 18A and the bottom coil 18B, each comprising two groups or columns of conductive strands 66 and a plurality of cooling ducts 67 disposed between each strand group. The cooling ducts 67 remove heat energy produced by current flow through the top and bottom coils 18A and 18B. As shown, the top and bottom coils 18A and 18B are separated by a void 68. Consolidation clips 70, typically constructed from copper, encircle and capture a conductive strand group at the end region of the conductive strands 66. Thus four consolidation clips 70 are shown in FIG. 3A. A similar arrangement of conductive strands, cooling ducts and consolidation clips is present at the opposing end of the top coil 18A and the bottom coil 18B.
It is known by those skilled in the art that other generator configurations comprise a stator coil including only a single coil such as the top coil 18A or the bottom coil 18B. In such a configuration only two consolidation clips are required, one consolidation clip for each strand group, with the two groups separated by cooling ducts. In still another configuration, a stator coil comprises only a single group of conductive strands, absent cooling ducts, with the strand group retained by one consolidation clip.
To form closed-circuit phase windings of the stator 14, the conductive strands 66 of the top coil 18A are electrically connected to the conductive strands 66 of the bottom coil 18B. Connected top and bottom coils 18A and 18B are then further connected to other interconnected top and bottom coils to form the closed-circuit stator phase windings. One known technique for effecting this connection between the top coil 18A and the bottom coil 18B brazes or solders an interconnecting copper bar 74 to opposing sides of both the top and bottom coils 18A and 18B. See FIGS. 4 and 5. An overlap region between the consolidation clip 70 and the copper bar 74 is indicated generally by reference character 76 in FIG. 4. Note from FIG. 5 that there are four such overlap regions, one on each opposing side of both the top coil 18A and the bottom coil 18B.
In certain coil embodiments, the overlap width is about one inch to about 1.25 inches, and the overlap length (designated “L” in FIG. 4) is dependent upon the coil height, (i.e., the distance between top coil 18A and the bottom coil 18B), which is typically in the range of about 3 inches to 5 inches. Assuming a coil height of 4 inches, each overlap region 76 presents an area of about 4 square inches. In certain other stator embodiments, the consolidation clip is replaced by a copper block that encircles the coil strands. Generally, the overlap region is larger in the embodiment employing the copper block.
After the brazing operation, the overlap region must be inspected to ensure that a high quality braze joint has been formed between the copper bar 74 and the consolidation clips 70. Inspection of the copper bar braze joints at the end of each stator coil is a critical element of generator installation. The inspection is advisable to determine the integrity of the braze joint and ensure that the performance of the generator will not be compromised by a braze joint failure. In addition to conducting an inspection during construction of the generator, the brazed joint is also inspected when a stator coil is rewound. An overlap inspection is also performed in those generator embodiments employing a copper block in lieu of a consolidation clip.
One prior art inspection process utilizes a stencil template in the form of a grid with quarter-inch grid squares for identifying individual inspection sites. An inspector places the stencil over the copper bar 74 in the overlap region 76, and using the grid squares as a guide, manually marks each inspection site to guide the subsequent inspection process. The stencil is removed and a couplant material (typically a gel-like substance) is applied to the copper bar 74 in the overlap region 70. An ultrasonic transducer is then manually positioned over each inspection site, as marked on the copper bar 74, for inspecting the quality of the braze joint at that site. The ultrasonic transducer emits ultrasonic energy (in one embodiment at about 2.25 MHz) and reads the echo return in each grid region. Differently sized transducers are available depending upon the area of the inspection region. For example, ultrasonic transducers having a diameter of 0.250″ and 0.375″ are available. Prior to beginning the inspection process, the surface of the copper bar 76 must be clean and free of any contaminants that can adversely affect the transmitted and received ultrasonic test signals.
If the copper bar 74 is not adequately brazed to the consolidation clip 70, an air pocket or void will be present between the mating surfaces. Since the void distorts the echo return, comparison of the actual return with a normal return from a properly mated surface allows void detection. Generally, a greater magnitude echo return indicates a void between the mating surfaces. The ultrasonic inspection process is based on a physical material property referred to as the acoustic impedance. Air has very high acoustic impedance and therefore incident ultrasonic energy is almost totally reflected (about 99.7% reflection) by air. A high quality brazed joint with no air voids between the mating surfaces produces a small echo return signal as most of the energy is absorbed by the brazed materials.
As ultrasonic energy is transmitted at each inspection site, the technician manually records the parameters of the echo return. After an entire overlap region 76 has been inspected, the number of problematic sites or the ratio of the problematic sites to the total number of sites is determined. In one inspection process, each inspection site is determined to either pass or fail the inspection based on the relationship between the return magnitude and a predetermined return threshold. The number of failed sites or the percent of failed sites to the total number of inspection sites is compared to a predetermined threshold, above which the brazed joint in that overlap region is considered unsatisfactory.
In one stator coil embodiment there are sixteen inspection sites in each overlap region 76. With four overlap regions on the top and bottom coils 18A and 18B, as illustrated in FIG. 5, there are 64 inspection sites for each coil end. Conventionally, a generator has 36 slots (see reference character 62 of FIG. 2) and thus 72 coils (a top and a bottom coil disposed in each slot), resulting in more than 1000 inspection sites. In addition to the large number of inspection sites, the prior art process is extremely tedious, as the inspector must manually reposition the ultrasonic transducer between the closely spaced inspection sites. One embodiment of the consolidation clip 70 has a width of about one inch, and is therefore segregated into three columns of 0.333-inch squares. Each square represents one inspection site. Another embodiment of a consolidation clip 70 has a width of about 1.25 inches, and is therefore segregated into three columns of 0.25 inch squares (i.e., there is one inspection site in each quarter-inch grid square). Thus it can be seen that manual positioning of the ultrasonic transducer in these small inspection sites requires the inspector to possess above average manual dexterity. The calculation for determining whether an inspection site has passed or failed is manually performed by the technician and thus subject to induced errors. There is no prior art process for automatically storing the return echo data.